The present invention is directed to intermetallic alloys for use as negative electrodes in hydrogen storage devices. Various intermetallic alloys have been widely used as negative electrodes in hydrogen storage devices for batteries. The nickel-metal hydride (Ni-MH) alkaline rechargeable battery, in which the positive electrode (cathode) is composed of nickel oxyhydroxide (NiOOH) and the anode is composed of a hydrided metal alloy, is the principle type of battery in which intermetallic hydrides are used.
The nickel-metal hydride battery satisfies the imminent need for environmentally-compatible rechargeable batteries superior to conventional nickel-cadmium (Ni--Cd) batteries in both versatility and performance. Nickel-metal hydride (Ni-MH) batteries have been developed by companies in the U.S. (Duracell) and Germany (Varta), and are in production by several companies in Japan (Sanyo, Matsushita Battery, Japan Storage Battery, Toshiba), Hong Kong (Gold Peak), and the U.S. (Eveready/Gates, Ovonic Battery, and Ovonic's licensee, Harding Energy Systems).
The Ni-MH electrochemical system utilizes a metal hydride (MH) anode in place of cadmium (Cd). Ni-MH batteries have operating characteristics similar to Ni--Cd. Both systems exhibit high rate discharge capabilities, fast charging, and similar voltage compatibilities. Likewise, both systems utilize similar charge cutoff methods and gas recombination mechanisms. The advantages of Ni-MH cells over Ni--Cd cells include: (1) higher specific energy (by a factor of 1.5-2.0 times), (2) higher energy density (160-200 Wh/l as compared to 70-120 Wh/l for Ni--Cd), (3) improved environmental compatibility, and (4) the promise of longer cycle life owing to the absence of life-limiting cadmium migration. The charge/discharge cycle lifetime of Ni-MH batteries has been limited by degradation of the metal alloy-based electrode.
An intermetallic alloy is considered suitable for use in an alkaline rechargeable battery if it satisfies two related physical criteria. First, the alloy should be able to absorb a large amount of hydrogen. Second, the alloy should also maintain a high degree of structural integrity and good hydrogen absorption characteristics over multiple charge/discharge cycles (e.g., hydrogen absorption/desorption). The first characteristic will be referred to as "capacity" or "hydrogen capacity," and the second characteristic will be referred to as "structural integrity" or "high cycle lifetime." The problem confronting designers of alkaline rechargeable batteries, therefore, is one of improving the structural integrity of the metal hydride alloy without sacrificing capacity.
The intermetallic alloys used in the anodes of alkaline rechargeable batteries include two generic classes: AB.sub.2 and AB.sub.5. In the AB.sub.2 category--alloys which adopt a Laves phase crystalline structure--several alloys have been used as hydrogen storage materials. A typical AB.sub.2 alloy may contain vanadium (V), titanium (Ti), nickel (Ni), zirconium (Zr), chromium (Cr) (e.g., V.sub.22 T.sub.16 Zr.sub.16 Ni.sub.39 Cr.sub.7). These different elements are assigned functional roles in hydrogen absorption/desorption (charge/discharge) cycling such as hydrogen storage (V, Ti, Zr), enabling the formation of a protective oxide (Ti and Zr) and embrittling the alloy hydride to produce high surface areas (Zr). As expected with the above-mentioned metal hydride alloy, the microstructure contains several (at least four) phases. The rationale that was used in designing these alloys was that short, medium, and long-range disorder in crystal structure and composition were beneficial to the cycle lifetime, and that these disorders allowed for good hydrogen-metal bond strengths, hydrogen storage capacity, and catalytic activity.
In the AB.sub.5 category--alloys which adopt a Haucke phase crystalline structure--most anode materials for Ni-MH batteries have been based upon LaNi.sub.5. This binary alloy offers relatively high capacity for hydrogen absorption (up to 400 mAh/g), relatively high rates of absorption, mild activation treatments, and low equilibrium pressures. The usefulness of LaNi.sub.5 as a metal hydride anode is limited since it suffers a rapid decline in capacity during charge/discharge cycling. This structural and functional degeneration of the alloy has been attributed to the formation of a thick layer of La(OH).sub.3, a process that is dependent on an intergranular diffusion of La to the surface of the electrode. The significant volume dilatation of the alloy upon hydriding (24%) causes pulverization, and is believed to promote the diffusion of elemental lanthanum.
Alloy modifications to LaNi.sub.5 have been made by substituting other elements for lanthanum, nickel, or both, and by changing the overall stoichiometry of the alloy.
The prevailing methodology used to slow this degradation has been to substitute small amounts of other elements for both Ni and La. Willems, et al. reported in J. Less Common Metals, 129, 13 (1987) improved alloy stability during electrochemical absorption-desorption cycling by a partial substitution of Si and Co for Ni, and of Nd for La. As reported in Proc. Symp. Hydrogen Storage Materials: Batteries and Electrochemistry, ECS Proc. Vol. 98-5, p. 59 (1992); J. Alloys and Compounds, 180, 37 (1992), Sakai, et al. performed a comprehensive evaluation of several different elements as substitutes in LaNi.sub.5-y M.sub.y where M corresponded to other transition metals. The cycle life was found to improve upon the substitution of Ni with the ternary solute in the order Mn&lt;Ni&lt;Cu&lt;Cr&lt;Al&lt;Co. In other studies, Sakai, et al. showed that substituting some solutes for La also enhances the cycle lifetime. In all the above ternary alloys, the improvement in the cycle lifetime is unfortunately accompanied by a decrease in the hydrogen absorption capacity, long activation, or slow kinetics.
While structural integrity has improved as a result of these substitutions, the beneficial lifetime effect was uniformly accompanied by a reduction in the hydrogen capacity of the metal alloy. It has also been found that substitution of other rare earth elements, such as cerium (Ce) and neodymium (Nd), for La improves the lifetime of Ni-MH batteries under charge/discharge cycling. These results were especially auspicious for mischmetal (Mm) substitutions for La in the LaNi.sub.5 -based ternary or higher-order alloys. Naturally-occurring Mm is comprised of a mixture of the early elements in the rare earth series. With Chinese Mm estimated to cost only about $5/kg, Mm substitutions for La would improve considerably the economics of AB.sub.5 battery electrodes and may be a desirable substitute to ensure commercial viability in the consumer marketplace.
A beneficial aspect of the Haucke phase structure is that it can be retained even when the ratio of B atoms to A atoms is different than 5.0, as discussed by K. H. J. Buschow and H. H. van Mal, J. Less Common Metals, 29 (1972), p 203. In stoichiometric AB.sub.5 alloys, the crystallographic A site is occupied exclusively by La and its substituents, and the B sites are occupied by Ni and its substituents. Buschow and van Mal have suggested that, when this ratio is less than 5, some B sites become occupied with A atoms. W. Coene, et al. found that when the ratio is greater than 5 and up to 6, depending on the particular B atom being substituted, some A sites are occupied with pairs of B atoms (Philosophical Magazine A, 65 (1992), p 1485). Notten, et al. (J. Alloys and Compounds, 210 (1994), p. 221) have produced electrochemical cells with off-stoichiometric Cu-substituted alloys. These examples show that the stoichiometry of Haucke phase metal hydride alloys does not have to be restricted to B/A =5.0. (Hereinafter, Haucke phase alloys included in the present invention will be referred to as either AB.sub.5 -based alloys or AB.sub.z alloys where 4.8.ltoreq.z6.0.)
It is unclear, however, what--if any--rationale has been used by the designers of the above-mentioned AB.sub.5 -based alloys. As noted above, various alloy modifications to LaNi.sub.5 have been made yet no guiding principle for the design of stable LaNi.sub.5 -based or MmNi.sub.5 -based ternary or higher order metal alloys has been devised. Currently; it is accepted that the structural integrity of AB.sub.5 and higher order AB.sub.5 -based alloys is determined primarily by the change in atomic volume during hydrogen absorption and that the deterioration of battery cells is controlled by the thermodynamic tendency to form oxides of the rare earth metal on the surface of the anode. In the analysis of AB.sub.5 and AB.sub.5 -based alloys, several competing theories regarding the structural integrity of metal alloys upon multiple cycles of hydrogen sorption have been advanced. Many of these substitution strategies appear to be based on a random sampling of candidate elements. None of these theories or explanations for improved structural integrity of these metal alloys can be applied uniformly to the variety of alloy compositions used in the anode of alkaline rechargeable cells. Furthermore, none accurately predicts the behavior of a class of metal hydride alloys upon hydriding and, therefore, none can be used to choose suitable candidate elements for substitution into a binary or higher order metal alloy.
It is clear, however, that ternary solute additions such as Sn can improve the cycle lifetimes of LaNi.sub.5 -based alloys during hydrogen absorption/desorption cycling but it is not clear why this is so. Furthermore, efforts to improve the structural integrity of metal alloys has resulted in a corresponding, and often overwhelming, loss of hydrogen capacity.
Mendelsohn, et al. (Inorg. Chem., 18, 3343 (1979) and Rare Earths in Modern Science and Technology, edited by G. J. McCarthy and J. J. Rhyne, Plenum, N.Y., 1980, p. 593) determined the reactivity of LaNi.sub.4.6 M.sub.0.4 alloys (where M=Al, Ga, In, Si, Ge, or Sn) with hydrogen gas. They reported that substitutions with these elements caused the unit cell volumes to enlarge which they correlated with decreases in the hydrogen desorption plateau pressures from the equilibrium pressure for LaNi.sub.5. These elements also produced undesirable reductions in total hydrogen storage capacities from that found with LaNi.sub.5. Although Si substitution significantly reduced hydrogen capacity, Meli, et al. have reported (in J. Alloys and Compounds, 190, 17 (1992) and J. Physk. Chem., 183, 371 (1994)) fast activation and very high stability upon electrochemical cycling of some Si substituted AB.sub.5 alloys.
The use of Sn as a partial substituent for Ni in LaNi.sub.5 was first reported by Mendelsohn, et al. in 1978 (Mat. Res. Bull., 13, 1221 (1979)). LaNi.sub.4.6 Sn,.sub.0.4 was found to have a plateau pressure of 0.076 atm, very low hysteresis, and an absorption capacity corresponding to a hydrogen-to-metal ratio of 0.97 (later measured to be 0.82 by S. Luo, et al., J. Alloys and Compounds (231, 467 (1995)). Tin also enhances the durability of the alloy, as evident in the works of Chandra, et al. (D. Chandra and F. Lynch, Rare Earths, The Metallurgical Society, Warrendale, Pa., 83 (1988)). The alloy LaNi.sub.4.8 Sn.sub.0.22 exhibited a capacity loss of only 15% after 10,000 gas-phase thermal absorption/desorption cycles. Our work showed the gas-phase cyclic lifetime of LaNi.sub.4.8 Sn.sub.0.2 to be 20 times greater than that of the binary alloy (as reported in R. C. Bowman, et al., J. Alloys and Compounds, 217, 85, (1995), incorporated herein by reference).
Thus, an object of the present invention is to identify ternary or higher-order alloys aimed at improving the structural integrity of the metal alloy over multiple charge/discharge cycles in an alkaline liquid medium, such as is the case for a rechargeable battery, while retaining high hydrogen capacity.
The present invention is also directed to electrode/catalyst materials for oxygen evolution/reduction reactions in batteries, fuel cells and electrolysers. The oxygen reduction reaction is utilized as the cathodic reaction in various electrochemical cells such as metal-air cells and fuel cells. The typical reactions for oxygen reduction are: EQU O.sub.2 +4H.sup.+ +4e.sup.- .fwdarw.H.sub.2 O) in acid EQU O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.- in base
The reduction of oxygen is routinely catalyzed by a variety of materials depending on the electrolyte and system temperature. Noble metals, and in particular platinum, have been traditionally used as the oxygen reduction catalyst in such systems. Platinum is normally dispersed on a high-surface area carbon support material to provide maximum capacity per unit mass of electrocatalyst.
A problem with using platinum as the oxygen reducing catalyst is that it is rather expensive. Thus, an object of the present invention is to identify relatively less expensive materials for use as an oxygen reducing catalyst in electrochemical devices and, in particular, fuel cells.